Control loops undergo a variety of conditions during their routine operation. For example, control loops in heating, ventilating, and air-conditioning (HVAC) systems for buildings may undergo setpoint changes, disturbances, slow drifts, etc. Characterizing the performance of control loops may ensure satisfactory operation of the control system, reduce variability and equipment wear, and detect problems such as oscillating behavior, sluggishness, and out-of-control situations.
Individual diagnostic testing functions exist for evaluating the performance of a control loop under particular operating conditions. For example, U.S. patent application Ser. No. 10/844,663 entitled “Method and Apparatus for Evaluating the Performance of a Control System,” filed May 13, 2004, which issued as U.S. Pat. No. 7,024,336 on Apr. 4, 2006; U.S. patent application Ser. No. 10/876,854 entitled “Method and Apparatus for Evaluating the Performance of a Control System,” filed Jun. 25, 2004; and PCT Application No. PCT/US2005/022127 entitled “Method and Apparatus for Evaluating the Performance of a Control System,” filed Jun. 22, 2005, published as WO 2006/012158 on Feb. 2, 2006 provide examples of individual diagnostic testing functions for evaluating the performance of a control loop. Each of these diagnostic testing functions may typically provide a diagnosis for a particular control loop condition (e.g., oscillating behavior, sluggishness, etc.) while ignoring situations for which it was not designed.
There is need for a state-based method and apparatus for evaluating the performance of a control system that combines diagnostic testing functions into a unified framework for evaluating control loop performance under different loop conditions, such as setpoint changes, disturbances, oscillations, normal operating conditions, etc.